In the construction of integrated circuit devices, a topside or “passivation” layer of a dielectric material is conventionally provided over the underlying layers comprising the integrated circuit structure. This layer, in addition to functioning as an insulation layer, acts to protect the underlying structure from moisture and ion contamination that can damage or destroy the structure by causing corrosion and electrical shorts.
Silicon nitride (Si3N4) is known as a satisfactory insulation material for forming such a passivation layer. Silicon nitride is known to have a high resistance to moisture and hydrogen penetration. Moreover, the diffusivity of various impurities, such as sodium, is much lower in silicon nitride than in other insulators, such as silicon dioxide. Thus, integrated circuits made with a silicon nitride passivation layer are less susceptible to ionic contamination problems.
In some memory integrated circuits, such as erasable programmable read only memories (EPROMs) and FLASH memories, ultraviolet (UV) light is irradiated on the topmost surface of the integrated circuit to erase underlying memory cells. In such devices, electric charge is stored on an electrically insulated floating gate. The UV light excites the electrons trapped on the floating gate, allowing them to escape off the floating gate to the overlying control gate or the underlying substrate.
The use of UV light to erase the internal memory cells of the integrated circuit implies that the passivation layer overlying the memory cells must be transmissive to the UV light, in order to allow the device to be erased properly. Silicon nitride films deposited by the usual method of plasma enhanced chemical vapor deposition (PECVD) are opaque to UV light. In the past, therefore, silicon nitride was considered unsuitable for use as the passivation layer on such devices, despite its outstanding performance as a moisture and contamination barrier.
Some practitioners have reported silicon nitride films that are transparent to UV radiation. However, a greater understanding of the reaction and the film is necessary, so that a better process and film may be created.